The pavement marking industry has long desired transparent, solid microspheres or beads that would be useful as brighter and/or more durable retroreflective lens elements in pavement markings. The transparent microspheres now most widely used for pavement markings are made of certain glasses, which are amorphous vitreous materials. Generally these glasses are of the soda-lime-silicate type having a refractive index of only about 1.5, which limits their retroreflective brightness. Glass microspheres can be scratched or chipped by grit and dirt particles on the highway.
Transparent microspheres are taught in U.S. Pat. No. 4,564,556. These microspheres are ceramic microspheres made by sol-gel processes from raw materials such as silica and zirconium compounds.
In order to function in pavement markings, such microspheres need to be resistant to scratching, chipping, cracking, and fracture under the conditions to which they are subjected on the road.